Rotella T6 5w-40 is what is most commonly used around here. 15w-40 in the heavily built (forged piston) or high mileage loose engines.
I use T6 5w-40 and it used to quiet my car down considerably (high mileage piston slapper). Not so much with the new formula, but its still a good synthetic for the price.
Another good option is Napas premium Synthetic in 10w-40.
I liked 15w-40 for the summer in my Forester, but the oil pump was really worn out and it needed heavier oil at startup to build pressure quicker. Sort of a false sense of security since Im sure the volume wasn't good, but it ran that way for the 20,000 miles I had it, and as far as I know, is still running...